In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube. The cladding tube is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods at a distance from each other and to prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each extending along the main part of the length of the fuel assembly.
Together with a plurality of other fuel assemblies, the fuel assembly is arranged in a core. The core is immersed into water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby, in a boiling water light-water reactor, part of the water is transformed into steam. The percentage of steam increases towards the top of the fuel assembly. Consequently, the coolant in the lower part of the fuel assembly consists of water whereas the coolant in the upper part of the fuel assembly consists both of steam and of water. This difference between the upper and lower parts gives rise to special problems which must be taken into consideration when designing the fuel assembly.
This problem can be solved by providing a flexible fuel assembly which in a simple manner may be given a shape where the upper part of the fuel assembly differs from the lower part so that optimum conditions may be obtained. A fuel assembly for a boiling water reactor with these properties is shown in PCT/SE95/01478 (Int. Publ. No. WO 96/20483). This fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section. A fuel assembly of this type may in a simple manner be given different designs in its upper and lower parts.
Also in a light-water reactor of pressurized-water type, it may be desirable to design the fuel assemblies so that each fuel assembly comprises a plurality of fuel units stacked on top of each other. As described above, each of the fuel units then comprises a plurality of fuel rods extending between a top nozzle and a bottom nozzle. A fuel assembly for a pressurized-water reactor, however, comprises no fuel channel.
One factor which must be taken into consideration when designing fuel units which are of the order of magnitude of 300-1500 millimeters long is that fission gases are formed during nuclear fission. In addition, the column of fuel pellets expands because of the heat formed in the fuel pellets. To take care of the fission gases and the thermal expansion of the column of fuel pellets, normally a relatively large space, an axial gap, is formed above the uppermost fuel pellet in the cladding tube in known full-length fuel rods, that is, fuel rods of the order of size of 4 metres long. The axial gap is of the order of size of 200-300 millimeters long. To this axial gap, the fission gases may thus diffuse and the column of fuel pellets may expand inwardly here. Thus, the axial gap contains no fissionable material.
Another factor which must be taken into consideration when designing axial gaps is that the temperature of the cladding tube in this region is lower than in the rest of the cladding tube because no fuel pellet is arranged in the axial gap. A problem which may arise as a result thereof is that hydrogen, formed, among other things, upon corrosion of the cladding tube, which is of a zirconium based alloy, and is absorbed thereby, will diffuse into this colder region. In the event that the concentration of hydrogen becomes too high in this region, hydrides are formed in the cladding material and cause embrittlement thereof. In a serious case, the cladding tube may burst and fissionable material may enter the cooling water. A tendency to the same type of problems may also appear in regions between the pellets, that is, where a lower end of a fuel pellet makes contact with an upper end of an adjacently located fuel pellet, and in the region between two fuel units stacked on top of each other. The risk of embrittlement because of too high a concentration of hydrogen increases, to a certain limit, with the size of the axial gap.
It is known to reduce the release of fission gas in different ways. One such way is to provide one or more of the fuel pellets with through-holes in their axial directions. In this way, the temperature in the fuel pellet is reduced, whereby the release of fission gas is reduced and the axial gap may be reduced. In this case, however, an axial gap of the order of size of a few millimeters is needed in a rod with a length of the order of size of 300 millimeters, up to a few tens of a millimeter for longer rods, to allow the thermal expansion of the column of fuel pellets.
The object of the present invention is to provide a fuel assembly for a light-water reactor with a plurality of short fuel units wherein at least one fuel assembly is designed with an axial gap and with means for taking care of hydrogen diffusing into this gap, thus preventing the build-up of impermissibly high concentrations of hydrogen in the material surrounding the axial gap.